This invention relates to scan conversion apparatus and a method for converting data sensed in sector format to raster format for display. More particularly, the invention relates to a scan converter and method for a single-sector or multi-sector ultrasonic scanner for sampling and storing the received echo data in a raster type geometry and processing the read-out data for display on a cathode ray tube (television monitor).
Conventional analog scan converters employ delicate electron beam storage tubes which are both expensive and difficult to maintain. Many previous attempts to implement a digital scan converter have either been very expensive or have introduced objectionable errors in the display resulting in degraded image quality. The basic reason for the poor image quality of such previous implementations is that the locations of the input data samples have not corresponded to those of the output data in a manner that permits a simple interpolation to obtain the correct output data. That is, the physical locations of the input data are not related to those of the output data of the scan converter in a simple way.
The single-sector scanner ultrasonic imager is a real time imaging system having a linear transducer array as depicted in FIG. 1. To make a sector scan the elemental transducers are excited in linear time sequence to generate angulated acoustic beams at many different angles relative to the normal to the array at the midpoint. Echoes returning from targets in the direction of the transmitted acoustic beam arrive at the transducer elements at different times necessitating relative delaying of the received echo electrical signals by different amounts to focus the received echoes, and the delayed echo signals are summed before being fed to the scan converter. It is common in prior art single-sector scanners to rotate the angulated transmitted beam by equal scan angle increments, and in the scan converter to sample the focused echo signals at equal time intervals so that the data samples are along arcs concentric to the origin point. The cathode ray tube, on the other hand, is a rectangular grid type display. The function of the scan converter is therefore relatively complex and a picture of uneven quality often results, worsened by the tendency of the eye to focus on uneven areas. A single-sector steered beam cardiac scanner with a TV monitor display is described by Thurstone and von Ramm in "A New Ultrasounc Imaging Technique Employing Two-Dimensional Electronic Beam Steering", Acoustical Holography, Vol. 5, 1974, Plenum Press, New York, pp. 249-259.
The present invention is applicable also to the multi-sector or "walking beam" ultrasonic imaging system having a longer linear transducer array for producing a set of sector scans with the points of the sequential sector scans displaced longitudinally along the array. This real time systrem capable of imaging randomly oriented targets and producing improved images is disclosed and claimed in U.S. Pat. No. 4,159,462, H. A. F. Rocha and C. E. Thomas, entitled "Ultrasonic Multi-Sector Scanner". A scan converter operative to store and read out only the largest echo amplitude at image points in overlapping areas of the sector scans is disclosed and claimed in allowed copending application Ser. No. 825,529 filed on Aug. 18, 1977 by E. T. Lynk entitled "Peak Detecting Digital Scan Converter" now U.S. Pat. No. 4,167,753. Both applications are assigned to the same assignee as this invention.